1. Field of the Invention
The present invention relates to a method of manufacturing a multilayered circuit board, by using dielectric composition, and more particularly, to an improved method of forming via holes and vias for connecting the layers constituting a multilayered circuit board.
2. Description of the Prior Art
Generally, a multilayered circuit board comprises conductive layers which are insulated by insulation films formed among them.
Methods of manufacturing multilayered circuit boards are roughly classified into the following two types: Method 1
The method comprises the steps of:
(1) forming via holes in predetermined portions of each insulating sheet made of insulative ceramic, by mechanical means such as a drill or a punch;
(2) filling the via holes with electrically conductive material;
(3) forming a conductor-pattern layer (i.e., an electric circuit) on each insulating sheet, in accordance with positions of the via holes;
(4) using positioning pins, thereby aligning the insulating sheets with the positioning holes or notches already cut therein, and then pressing and forming the insulating sheets together; and
(5) firing the multilayered structure, thus pressed and formed, simultaneously. Method 2
This method comprises the steps of:
(1) forming a conductive layer by using polymerbase material for forming thick film, on a substrate made of ceramic dielectric material such as alumina, by means of screen-printing, and firing to remove volatile solvent, and to sinter the solid component;
(2) forming a required number of via holes in predetermined portions of insulating sheet, by mechanical means such as a drill or a punch;
(3) laminating the insulating sheet of the step (2) on, and precisely aligning the insulating sheet with, the substrate made by the step (1), and pressing, and firing the resultant structure;
(4) filling the via holes with conductive paste for forming thick film, by means of screen printing, and firing the conductive paste filled in the via holes;
(5) forming a conductor-pattern layer (i.e., an electric circuit) on the fired insulating sheet in accordance with the registration of the vias, and firing; and
(6) repeating the steps (2) to (5), thereby forming a desired number of other conductor-pattern layers.
Methods 1 and 2 have their respective characterizing features, in the way how the components of the circuit board shrink while being fired. Method 1 is to form a multilayered structure comprising a plurality of insulating sheets and a plurality of conductor-pattern layers. When fired, they each are sintered, and they shrink in three dimensions, forming an integral multilayered circuit board (integral-type circuit board). In method 2, a plurality of insulating sheets are laid on the substrate, one upon another. The substrate does not shrink when the whole structure is fired. Only the multilayered structure on the substrate shrinks in the direction of thickness, whereby a multilayered circuit board fixed to a substrate is provided (composite type circuit board).
Methods 1 and 2 have drawbacks which result from their respective characterizing features. Method 1, wherein insulating sheets are laid one upon another, is disadvantageous in the following respects:
a) A jig is required for aligning the insulating sheets, and it is difficult to align the sheets with precision.
b) High-precision process is required to fill the via holes with conductive material, and the yield of the circuit board is low if the circuit formed thereon has a high integration density.
Method 2, wherein the insulating sheet having via holes is laid on a substrate with a conductor-pattern layer on it, and then the sheet and the substrate are fired, is disadvantageous in the following respects:
c) The insulating sheet having via holes must be precisely aligned with the conductor-pattern layer formed on the substrate. This alignment inevitably required a complex and, hence, expensive apparatus, and results in a low yield of the circuit board.
d) The shrinking of the insulating sheet, resulting from the firing, adversely influences the alignment between the sheets.
Moreover, methods 1 and 2 have the following drawback in common:
e) Since each via hole cut in the insulating sheet by the mechanical means has a cylindrical shape, air is likely filled in the hole along with the conductive material. This jeopardizes the electrical connection between the conductor-pattern layers achieved by the via.
Because of the drawbacks mentioned above, the multilayered circuit boards which can be made by either method have some limitations. For example, the circuit boards cannot be sufficiently dense in a Z axis (i.e., in the direction of lamination), or the insulating sheets incorporated in the boards must have a thickness falling within a specific range (e.g., 300 to 600 .mu.m), so that the sheet can be easily processed.